1. Field of the Invention
This invention relates to hydraulic systems, and more particularly to a hydraulic system combining open center valve circuitry and closed center valve circuitry in a single system, and in which a priority hydraulic flow is insured for the circuit which includes the open center control valve. The hydraulic system of the invention may be used in a number of practical embodiments and will be described in the present application as embodied in a hydraulic control system for an agricultural apparatus such as a combine having a plurality of components which are hydraulically controlled.
2. Description of the Prior Art
By way of definition it might be explained that it is well known in the hydraulic control system art that an "open center" control valve is one which has a hydraulic fluid flow therethrough even when the open center control valve is in neutral position, and that such open center control valves have utility, for example, particularly in a system such as certain steering control systems for agricultural vehicles in which the steering system of the vehicle requires a priority minimal hydraulic fluid flow at all times through the control valve thereof even when the steering device is not being actuated for a turning movement.
By way of definition it is also well known in the prior art that a "closed center valve system" is a hydraulic control system in which there is no hydraulic flow through the control valve when the control valve is in its neutral position.
It has been known in the prior art relating to hydraulic control systems for large agricultural machines such as combines which have a plurality of hydraulically operated components to provide two separate pumps for supplying the hydraulic fluid to the various hydraulically operated components and to the hydraulic control circuits thereof. One pump, for example, may be used to provide hydraulic flow to an open center spool valve bank which provides (1) header lift control; (2) reel lift control; (3) reel speed control; (4) ground speed control. In the prior art a separate pump is usually provided for the hydraulic requirements of the steering system of such agricultural vehicles.
The use of multiple pumps to perform the plurality of hydraulic functions associated, for example, with large agricultural apparatus such as a combine, is expensive, and the open center hydraulic control systems with which such multiple pumps are usually associated waste power since as previously explained an open center hydraulic control system inherently includes a hydraulic flow through the valve system even when the valve is in a neutral position.